1. Field of the Invention
This invention relates to Group III (Al, Ga, In)N articles (e.g., crystals, boules, substrates, wafers, layers, films, and the like) useful for producing optoelectronic devices (such as light emitting diodes (LEDs), laser diodes (LDs) and photodetectors) and electronic devices (such as high electron mobility transistors (HEMTs)) composed of III-V nitride compounds, and to methods for producing such articles.
2. Description of the Related Art
Group III-V nitride compounds such as aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN), and alloys such as AlGaN, InGaN, and AlGaInN, are direct bandgap semiconductors with bandgap energy ranging from about 0.6 eV for InN to about 6.2 eV for AlN. These materials may be used to produce light emitting devices such as LEDs and LDs in short wavelength in the green, blue and ultraviolet (UV) spectra. Blue and violet laser diodes may be used for reading data from and writing data to high-density optical storage discs, such as those used by Blu-Ray and HD-DVD systems. By using proper color conversion with phosphors, blue and UV light emitting diodes can be made to emit white light, which may be used for energy efficient solid-state light sources. Alloys with higher bandgaps may be used for UV photodetectors that are insensitive to solar radiation. The material properties of the III-V nitride compounds are also suitable for fabrication of electronic devices that may be operated at higher temperature, higher power, and higher frequency than conventional devices based on silicon (Si) or gallium arsenide (GaAs).
Most of the III-V nitride devices are grown on foreign substrates such as sapphire (Al2O3) and silicon carbide (SiC) because of the lack of available low-cost, high-quality, large-area native substrates such as GaN substrates. Blue LEDs are mostly grown on insulating sapphire substrates or conductive silicon carbide substrates using a metal-organic chemical vapor deposition (MOCVD) process. Sapphire belongs to the trigonal symmetry group, while SiC belongs to the hexagonal symmetry group. GaN films and InGaN films have been heteroepitaxially grown on the c-plane sapphire surface for LED devices. Due to lattice mismatch, the GaN films grown on both sapphire and SiC substrates typically have high crystal defects with a dislocation density of 109 to 1010 cm−3. Despite the high defect density of the LEDs grown on these substrates, commercial LEDs have long lifetimes suitable for some applications.
The MOCVD process is a slow growth rate process with a growth rate of a few microns per hour. In a typical GaN-based device growth process, a low-temperature buffer layer of GaN or AlxGa1-xN (x=0-1) is first grown on a foreign substrate (e.g., sapphire or silicon carbide), followed by the growth of a few microns of GaN. The active device layer, such as quantum well structures for LEDs, is subsequently grown. For example, U.S. Pat. No. 5,563,422 to S. Nakamura et al. describes a GaN-based device grown by an MOCVD process. A thin GaN nucleation layer of about 10 nanometers is first deposited on a sapphire substrate at a low temperature of 500-600° C. The GaN nucleation layer is annealed at high temperature to recrystallize the GaN, and epitaxial GaN film is grown at higher temperature (approximately 1000-1200° C.).
UV LEDs based on alloys of GaN, however, show strong dependence of the power output on the substrate material used. The UV LEDs can be grown on native GaN substrates or on foreign substrates such as sapphire and silicon carbide. On the foreign substrate, a GaN or AlGaN thin film is first grown by utilizing appropriate techniques and the active UV LED structure is subsequently grown. It has been found that the power output of UV LEDs grown on native GaN substrates is much greater than the power output of those grown foreign substrates (see, for example, Yasan et al. Applied Physics Letters, Volume 81, pages 2151-2154 (2002); Akita et al. Japanese Journal of Applied Physics, Volume 43, pages 8030-8031 (2004)). The lower density of crystal defects in the device structure grown on a native GaN substrate contributes to higher power output.
Group III-V nitride-based laser diodes also show a remarkable dependence of lifetime on the crystal defect density. The lifetime of the LDs dramatically decreases with the increase of the dislocation density (see, for example, “Structural defects related issues of GaN-based laser diodes,” S. Tomiya et al., MRS Symposium Proceedings, Vol. 831, p. 3-13, 2005). Low defect density single-crystal gallium nitride substrates are needed for the long lifetime (>10,000 hours) nitride laser diodes. For LEDs based on an AlGaN active layer operating at the deeper UV range, it is also found that dislocation density has a detrimental effect on the performance and lifetime of the devices. For LEDs operating at higher power levels, it is also desirable to have a lower defect density GaN layer.
Because of the very high equilibrium nitrogen pressure at the melting point, gallium nitride single crystals cannot be grown with conventional crystal growth methods such as the Bridgman method or Czochralski method where single crystals are grown from the stoichiometric melt. At ambient pressure, GaN starts to decompose well before melting.
Despite such difficulties, small-size high-quality single-crystal GaN substrates have been made with several methods. Porowski et al. discloses a method of growing bulk GaN at high nitrogen pressure (S. Porowski and I. Grzegory, J. Cryst. Growth, Vol 178, 174 (1997)). Gallium metal is reacted with gaseous nitrogen at a nitrogen pressure as high as 20 kbar and a temperature as high as 2000 K. The crystal growth rate of the process is slow and long growth times of 60-150 hours are needed to produce crystal platelets of about 1 cm in length and about 0.1 mm in thickness. It is also believed that the process is not scalable, i.e., longer growth times or larger reactors cannot increase the crystal size substantially. Disalvo et al. in U.S. Pat. No. 5,868,837 discloses a method of GaN growth using sodium flux where gallium metal reacts with gaseous nitrogen under moderate temperature and pressure. However, the GaN crystals grown with the sodium flux method are small, about a few millimeters in size. D'Evelyn et al. in U.S. Pat. App. Pub. Nos. 2004/0124434 and 2005/0098095 discloses the growth of GaN in supercritical ammonia (NH3). Although high crystal quality can be achieved, the growth rate may be quite slow, and thus produces crystals with small area. Therefore, GaN crystal growth from liquid phase, in general, yields small crystals, and is not suitable for commercial applications.
Large-area GaN crystals may be grown by hydride vapor phase epitaxy (HVPE) methods. In the HVPE process, gallium chloride (GaCl), formed by reacting gaseous hydrochloric acid (HCl) with gallium metal in the upstream of the reactor, reacts with ammonia (NH3), depositing GaN on the surface of a substrate. The size of the GaN crystal grown may be the same as the size of the substrate. Substrates such as sapphire, gallium arsenide, silicon carbide, and other suitable foreign substrates have been used. Since large-sized substrates with a diameter from 2 inches to 12 inches are available, large-sized GaN, in theory, could be grown with HVPE techniques. However, bulk GaN growth on sapphire substrate by HVPE encounters many obstacles, such as nucleation of polycrystalline material, cracking and microcracking during growth and cool down, and an unstable crystal growth front that leads to polycrystalline formation or microcracking during the bulk growth.
Bulk GaN growth methods, in which multiple wafers can be produced, can significantly reduce the cost the GaN wafer manufacturing. The material quality also improves with the bulk growth. Vaudo et al. in U.S. Pat. No. 6,596,079 discloses a vapor phase method for growing a GaN boule using native GaN crystal as a seed. However, to practice the invention, a high-quality GaN seed is first required. Melnik et al. in U.S. Pat. No. 6,616,757 discloses a similar method for growing GaN crystal boule by hydride vapor phase epitaxy. A GaN single-crystal layer is first grown on a silicon carbide substrate, the substrate is subsequently removed by etching in molten KOH to form a GaN seed, and a GaN boule is grown on the seed to a length greater than 1 centimeter. However, silicon carbide substrates are of higher cost than sapphire substrates and the process is not applicable to sapphire substrate since sapphire cannot be etched away. Motoki et al. in U.S. Pat. Nos. 6,413,627; 6,468,347; and 6,667,184 discloses methods for growing a bulk GaN crystalline boule by HVPE on GaAs and on native GaN seeds. Because of the processes used, the GaN crystal boule grown by Motoki's methods contain clusters of highly defective material.
In view of such prior-art approaches to bulk GaN crystal growth, it is well-acknowledged that there is still a need in the art for low-cost methods for manufacturing high-quality GaN crystal boules and wafers.
It is also of interest to produce free-standing GaN articles by separating or removing the underlying substrate. Vaudo et al. in U.S. Pat. No. 6,440,823 discloses a method for producing low defect GaN using HVPE on sapphire substrates. The sapphire substrate can be removed to produce large-area GaN substrate, for example, by a laser induced liftoff process as described by Kelly et al. (“Large freestanding GaN substrates by hydride vapor phase epitaxy and laser-induced liftoff,” Jpn J. Appl. Phys., Vol. 38, L217-L219, 1999). The wavelength of the laser beam, or the energy of the laser beam, is chosen so that it is smaller than the bandgap of the substrate, but larger than the bandgap of GaN. The substrate is transparent to the laser beam, but the GaN absorbs the laser energy, heating the interface and decomposing the GaN at the interface, which separates the GaN film from the substrate. In U.S. Pat. App. Pub. No. 2002/0068201, Vaudo et al. further discloses a method for producing freestanding GaN near the growth temperature by shining a laser beam at the interface between the grown GaN layer and the template, and decomposing the interface material. This process involves dangerous high-energy laser beams and high manufacturing cost. Park et al. in U.S. Pat. No. 6,652,648 discloses a similar method for producing GaN substrate by first growing HVPE GaN on sapphire substrates and followed by laser liftoff. Motoki et al. in U.S. Pat. No. 6,693,021 discloses a method for growing a thick GaN film on a gallium arsenide (GaAs) substrate, in which the GaAs substrate was wet-etched away to produce a free-standing GaN substrate.
The known methods for producing GaN substrates such as disclosed in U.S. Pat. Nos. 6,693,021; 6,652,648; and 6,440,823 can only yield one wafer per growth run, and thus are of high manufacturing cost.
Chin Kyo Kim in U.S. Pat. No. 6,923,859 discloses an apparatus and associated manufacturing method for GaN substrates in which a substrate and a GaN layer are separated from each other after growing the GaN layer on the substrate in the same chamber. The apparatus contains a transparent window at the circumference of the chamber to allow the introduction of the laser beam to the substrate. This process likewise involves dangerous high-energy laser beams and has high manufacturing cost.
Bong-Cheol Kim in U.S. Pat. 6,750,121 discloses an apparatus and method for forming a single crystalline nitride substrate using hydride vapor phase epitaxy and a laser beam. After growth of the GaN film on sapphire substrate, the wafer is moved to a heated chamber for laser-introduced separation. Because the wafer does not cool to room temperature, cracking induced by the mismatch of the coefficient of thermal expansion is eliminated. This process likewise involves dangerous high-energy laser beams and has high manufacturing cost.
Park et al. in U.S. Pat. No. 6,652,648 discloses a method for producing GaN substrate by first growing HVPE GaN on sapphire substrates. The backside of the sapphire substrate is protected for minimal parasitic deposition. After GaN growth on sapphire substrate, the GaN/sapphire structure is removed from the reactor. The GaN layer is subsequently separated from the sapphire substrate by a laser liftoff process. In addition to involving dangerous high-energy laser beams, the GaN layer on sapphire is likely to crack upon cool down, and thus this process suffers with low yield and high manufacturing cost.
Motoki et al. in U.S. Pat. No. 6,693,021 discloses a method of growing thick GaN film on gallium arsenide (GaAs) substrate. The GaAs substrate is wet-etched away to produce a free-standing GaN substrate. However, GaAs substrates tend to thermally decompose at the GaN growth temperature and in the GaN crystal growth environment, introducing impurities to the GaN film.
Yuri et al. in U.S. Pat. No. 6,274,518 discloses a method for producing a GaN substrate. A first semiconductor film (AlGaN) layer is formed on a sapphire substrate, and a plurality of grooves is formed on the AlGaN layer. A relatively thick GaN film is grown on a grooved AlGaN template by an HVPE method, and upon cooling down from the growth temperature to room temperature, GaN separates from the template, forming a large area freestanding GaN substrate. However, this method requires deposition and patterning of several films in different systems and cracking-separation. Thus, the process is one of low yield and high manufacturing cost.
Solomon in U.S. Pat. No. 6,146,457 discloses a thermal mismatch compensation method to produce a GaN substrate. The GaN film is deposited at a growth temperature on a thermally mismatched foreign substrate to a thickness on the order of the substrate, where the substrate is either coated with a thin interlayer or patterned. After cool down from the growth temperature to the room temperature, it is stated that thermal mismatch generates defects in the substrate, not in the GaN film, producing a thick high-quality GaN material. However, the GaN material of Solomon's invention is still attached to the underlying substrate, with the underlying substrate containing substantial defects and/or cracks. Subsequently, other processing steps are required to create a freestanding GaN layer.
Usui et al. in U.S. Pat. No. 6,924,159 discloses a void-assisted method for manufacturing GaN substrate. In this method, a first GaN thin film is deposited on a foreign substrate, and a thin metal film such as titanium film is then deposited on the first GaN thin film. The titanium metal film is heated in hydrogen-containing gas to form voids in the first GaN thin film. A thick GaN film is subsequently deposited on the first void-containing GaN film. The voids in the first GaN film create fracture weakness, and upon cooling from the growth temperature to ambient room temperature, the thick GaN layer separates itself from the substrate, forming a GaN wafer. However, this method requires deposition of several layers of films in different systems and cracking-separation. Thus, the process is one of low yield and high manufacturing cost.
The techniques of the prior art for manufacturing GaN wafers are attended by high manufacturing cost. There are some commercial vendors currently selling 2″ GaN wafers, but at very high price, reflecting the high manufacturing cost. Additionally, researchers have shown that the freestanding GaN substrates formed using the laser-induced liftoff process can be subject to substantial bowing, which limits their usability for device manufacturing (see, for example, “Growth of thick GaN layers with hydride vapour phase epitaxy,” B. Monemar et al., J. Crystal Growth, 281 (2005) 17-31).
In view of such prior-art approaches to forming GaN substrates, it is well-acknowledged that there is still a need in the art for low-cost methods for producing high-quality free-standing GaN substrates.
There is also interest in fabricating electronic devices in which an active layer is built on GaN film. Because of the lattice mismatch between gallium nitride and the non-native substrate, there is a large number of crystal defects in the GaN film and active device layer. The defect density in the GaN nucleation layer is thought to be on the order of 1011 cm−2 or greater, and in the subsequently grown GaN layer and active device layer, the typical density of crystal defects, in particular, the threading dislocation density, is on the order of 109-1010 cm−2 or greater in typical GaN-based LEDs as previously noted. Moreover, because of the large mismatch in both the thermal expansion coefficients and the lattice constants of the foreign substrate and the GaN film, problems such as a high defect density lead to short device lifetime and bowing of GaN/heteroepitaxial substrate structures. Bowing leads to difficulty in fabricating devices with small feature sizes.
For LEDs based on an AlGaN active layer operating at the deeper UV range, it is also found that dislocation density has a detrimental effect on the performance and lifetime of the devices. For LEDs operating at higher power levels, it is also desirable to have a lower defect density GaN layer.
There are several growth methods that may possibly be performed to reduce the defect density of the gallium nitride film. One common approach in MOCVD growth of gallium nitride is epitaxial lateral overgrowth (ELOG) and its variations. In an ELOG GaN growth process, a GaN film is first grown by a MOCVD method with the 2-step process (low-temperature buffer and high-temperature growth). A dielectric layer such as silicon oxide or silicon nitride is deposited on the first GaN film. The dielectric layer is patterned with a photolithographic method and etched so that portions of GaN surface are exposed and portions of the GaN film are still covered with the dielectric mask layer. The patterned GaN film is reloaded into the MOCVD reactor and growth is re-commenced. The growth condition is chosen such that the second GaN layer can only be grown on the exposed GaN surface, but not directly on the masked area. When the thickness of the second GaN layer is thicker than the dielectric layer, GaN can grow not only along the original c direction, but also along the sidewalls of the GaN growing out of the exposed area and gradually covering the dielectric mask. At the end of the growth, the dielectric mask will be completely covered by the GaN film and the GaN film overall is quite smooth. However, the distribution of the threading dislocation density is not uniform. Since the dislocation density of the first GaN layer is quite high, the defect density is also high in the area of the second GaN layer grown directly on the exposed first GaN layer. In comparison, the defect density is much reduced in the area above the dielectric mask area where the second GaN layer was grown laterally in the direction parallel to the surface. The defect density is still high in the area where the second GaN layer was grown directly on the first GaN layer and in the area where the lateral grown GaN coalesced. Multiple ELOG processes can be used to further reduce the defect density by patterning a second dielectric mask covering the high defect density areas of the first ELOG GaN film, and growing GaN film in the ELOG condition that yields a coalesced second ELOG film.
The manufacturing cost of the prior-art low defect density GaN film based on MOCVD is high due to multiple growth and photolithographic steps. The high cost of the film also increases the overall manufacturing cost of end products such as UV LEDs.
Therefore, there is still a compelling need in the art for low-cost methods for producing high-quality, low defect density GaN films that are suitable for electronic and optoelectronic devices to be built on.
Conductive GaN substrates have recently become available. Such conductive GaN substrates are advantageously employed in the manufacturing of blue and UV lasers. However, in a number of electronic applications such as high electron mobility transistors (HEMTs), an insulating or semi-insulating GaN substrate is highly desirable.
Unintentionally doped GaN exhibits n-type conductivity due to the presence of residual n-type impurities as well as crystal defects. Since GaN has a high bandgap energy, a pure and defect-free GaN material should exhibit insulating or semi-insulating electric properties. However, current GaN crystal growth techniques still allow the unintentional incorporation of impurities and various crystal defects such as vacancies and dislocations, which render the GaN crystals conductive.
It is known in the prior art that by introducing deep-level compensating impurities in the crystal, a wide bandgap semiconductor can be made semi-insulating. For example, U.S. Pat. No. 5,611,955 issued to Barrett et al. discloses the use of vanadium doping in silicon carbide to produce a semi-insulating SiC crystal. Similarly, Beccard et al. discloses the use of iron chloride formed by reacting elemental iron with gaseous hydrochloric acid in a vapor phase reactor during the HVPE growth of indium phosphide (InP) to produce iron-doped semi-insulating InP crystals (R. Beccard et al., J. Cryst. Growth, Vol. 121, page 373-380, 1992). The compensating impurities act as deep-level acceptors to trap the otherwise free electrons generated by unintentionally doped n-type impurities and crystal defects. The concentration of the deep-level acceptor is typically higher than the concentration of the free electrons generated by the n-type impurities and crystal defects.
Several deep-level acceptors generated by compensating impurities in gallium nitride (GaN) have been identified in the prior art. For example, Group II metals such as Be, Mg, and Zn, and transition metals such as Fe and Mn, can be incorporated in the GaN crystal resulting in semi-insulating electric properties. The energy level of iron in gallium nitride is well-documented and iron incorporation can result in gallium nitride exhibiting the semi-insulating electric property (see, for example, R. Heitz et al., Physical Review B, Vol. 55, page 4382, 1977). Iron-doped gallium nitride thin films can be grown with metal-organic chemical vapor deposition, molecular beam epitaxy, and hydride vapor phase epitaxy (see, for example, J. Baur et al., Applied Physics Letters, Vol. 64, page 857, 1994; S. Heikman, Applied Physics Letters, Vol. 81, page 439, 2002; and A. Corrion, et al., Journal of Crystal Growth, Vol. 289, page 587, 2006). Zinc-doped gallium nitride thin films grown by hydride vapor phase epitaxy can be semi-insulating as well (N. I. Kuznetsov et al., Applied Physics Letters, Vol. 75, page 3138, 1999).
U.S. Pat. No. 6,273,948 issued to Porowski et al. discloses a method for making a highly resistive GaN bulk crystal. The GaN crystal is grown from molten gallium under an atmosphere of high-pressure nitrogen (0.5-2.0 GPa) and at high temperature (1300-1700° C.). When pure gallium is used, the GaN crystal grown is conductive due to residual n-type impurities and crystal defects. When a mixture of gallium and a Group II metal such as beryllium, magnesium, calcium, zinc, or cadmium is used, the grown GaN crystal is highly resistive, with a resistivity of 104-108 ohm-cm. However, the crystals obtained from molten gallium under the high-pressure, high-temperature process were quite small, on the order of one centimeter, which is not suitable for most commercial electronic applications.
U.S. Pat. App. Pub. No. 2005/0009310 by Vaudo et al. discloses a large-area semi-insulating GaN substrate grown by hydride vapor phase epitaxy (HVPE). Typically, undoped HVPE-grown GaN is of n-type conductivity due to the residual impurities and crystal defects. By introducing a deep-level doping species during the growth process and at a sufficiently high concentration of the dopant species in the GaN crystal, the grown GaN crystal becomes semi-insulating. Typical dopant species are transitional metals such as iron.
However, during the HVPE growth of single-crystal GaN, there are various surface morphologies observed and these different growth morphologies have different levels of impurity incorporation. U.S. Pat. No. 6,468,347 by Motoki et al. discloses that in the growth of GaN on c-plane substrate by HVPE, the growth surface has inverse pyramidal pits. Because of the presence of the pits on the growing GaN surface, the actual GaN growth takes place both on the non-pitted area, which is normal c-plane growth, and on the faces of the pits, which is non-c-plane growth. U.S. Pat. Nos. 6,773,504 and No. 7,012,318 by Motoki et al. disclose that GaN growth on the surfaces other than the c-plane has much higher oxygen incorporation.
The presence of inverse pyramidal pits on the GaN crystal surface during HVPE growth results in a non-uniform distribution of n-type impurity concentration in the GaN crystal due to higher impurity incorporation on the non-c-plane surfaces. The impurity concentrations in these pitted areas can be an order of magnitude or more higher than in non-pitted areas. Even when compensating deep-level impurities such as iron are introduced during the crystal growth, the electric characteristics of the grown GaN crystal are not uniform when pits are present during the growth, and GaN wafers made from such crystals will have a non-uniform sheet resistance across the wafer surface. When the as-grown GaN is polished to remove the pits and to produce a smooth surface, the impurity concentration on the surface is still not uniform. The areas where pits were present have a higher oxygen impurity concentration, appear to be darker in color than the surrounding area, and are considered as “inclusions” of more conductive spots. Electronic devices grown on substrates with non-uniform electric properties have lower performance, resulting in lower device yield. Substrates that are “inclusion-free,” or those substrates without non-uniform areas of more conductive spots, would have a more uniform sheet resistance across the wafer surface and higher performance, resulting in higher device yield.
Therefore, there is also a compelling need in the art for large-area, inclusion-free, uniform semi-insulating GaN substrates and methods for making such substrates.